C#

//input validation already done here<br />
.<br />
.<br />
.<br />
<br />
if (double.Parse(lblAmtMax.Text) < double.Parse(txtAmount.Text) || double.Parse(lblBalance.Text) < 0)<br />
{<br />
    lblBalance.Text = "0.00";<br />
    txtAmount.Text = lblAmtMax.Text;<br />
    return;<br />
}<br />
<br />
//get diffce between amtEntered and last Amt entered then add it(positive or negative) to existing balance<br />
double diffce = double.Parse(lblBalance.Text) + (double.Parse(lastEnteredAmount) - double.Parse(txtAmount.Text));<br />
lblBalance.Text = diffce.ToString();<br />
lastEnteredAmount = txtAmount.Text;